1. Field of the Invention
The present invention relates to a failure detecting device which is capable of detecting a failure of a power supply changeover switch such as sticking of contacts of the switch.
2. Description of the Related Art
In a power supply system provided with an electric generator and a battery charged by the generator, such as a power supply system for a vehicle, a power supply changeover switch (hereinafter referred to as "a changeover switch") is used for supplying electric power to a specific electric load (a load such as an electric heater which consumes a large electric power). In this type of the system, the changeover switch disconnects the battery from the generator and directly connects the large electric load to the generator so that electric power is directly supplied from the generator to the large electric load.
This type of the power supply system is, for example, disclosed in Japanese Unexamined Patent Publication (Kokai) No. 52-111131. The '131 publication discloses a power supply system for a vehicle which supplies electric power to an electric heater of a defogger for removing fog and ice from a windshield of the vehicle. In the system of the '131 publication, the defogger is directly connected to a generator of the vehicle when the defogger is activated, and a battery is disconnected from the generator in order to supply a large electric power to the defogger.
In general, when a battery is connected to a generator, an output voltage of the generator is controlled to a predetermined battery charging voltage in order to prevent an overcharging of the battery. Therefore, if a large electric load such as a defogger and the battery are connected to the generator at the same time, the output voltage of the generator must be controlled to a relatively low value (i.e., the charging voltage). In this case, since the output voltage of the alternator is low, the electric current and electric power supplied to the electric load also becomes low. This causes insufficient heat generation of the heater in the defogger and a time required for removing fog and ice is prolonged. In the system of the '131 publication, it may be possible to set the resistance of the heater in the defogger at a low value to supply a large electric power to the defogger even in a low voltage condition. However, in this case, since the output power of the generator is relatively low due to its low output voltage, the generator cannot supply sufficient electric power to the defogger, and the battery must supply electric power to the defogger in order to make up the shortage of electric power required by the defogger. Therefore, a wear of the battery may occur due to an increased load of the battery.
In order to solve this problem, the system in the '131 publication disconnects the battery from the generator when the defogger is activated so that the output voltage of the generator can be raised. By raising the output voltage, since the output power of the generator increases accordingly, a sufficient electric power is supplied to the defogger. Further, since the battery is disconnected from the generator, the overcharging of the battery does not occur even though the output voltage of the generator is raised. Namely, in the system of the '131 publication, a large electric power can be supplied to a specific load without increasing the load of the battery.
In the system which selectively connects a battery and a specific load to the generator as disclosed in the '131 publication, a changeover switch is used for switching the connections between the battery, the specific load and the generator. In general, a large capacity mechanical switch (relay switch) is used in order to switch a large electric current. However, since a large electric current must be cut off instantaneously in the changeover switch, sticking of the switch sometimes occurs due to welding of the contacts or other causes. If the sticking of the contacts of the changeover switch occurs at the position where it connects the electric load to the generator, for example, the electric load may be overheated due to an excessive power supply and, since the battery is not charged by the generator, an excessive discharge of the battery may occur. Further, if the sticking of the changeover switch occurs at the position where it connects the battery to the generator, electric power is not supplied to the load and, in addition to that, overcharging of the battery may occur due to the raised output voltage of the generator.
Therefore, it is important to detect the failure of the changeover switch such as the sticking of the contacts when the changeover switch is used in the power supply system. Although it is important, no consideration is given to the problem of detecting the failure of the changeover switch in the '131 publication.